In a heat treatment furnace that heat treats a treatment object, a heater for heating the furnace atmosphere is provided. As the heater used in the heat treatment furnace, for example, Patent Document 1 describes a sheet metal heater to be used in a continuous heat treatment furnace. Further, Patent Document 2 describes a Kanthal (registered trademark) heater arranged along an inner wall of a heating furnace. Patent Document 3 describes a heater including a U-shaped heating part, or a bellows-shaped heater including a continuous U-shaped heating part. Patent Document 4 describes a bellows-shaped heater provided so as to be horizontally inserted into a heating furnace from its side wall. As above, there are various types of heaters as the heater for a heat treatment furnace.
Such heaters as described above are employed also for a carburizing furnace that carburizes a low-carbon steel workpiece. It is general that a furnace wall of the carburizing furnace is composed of an outer wall (iron shell) and a plurality of heat insulators. The heater for a carburizing furnace is arranged to face the heat insulator located at the innermost of the furnace wall (to be referred to as a “first heat insulator” hereinafter).
However, the heater has a structure to emit heat radially from the heating part, and thus emits heat to the outer wall side as well as to the furnace inner side. That is, the heat is supplied also to the above-described first heat insulator, resulting in that the surface temperature of the furnace inner side of the first heat insulator becomes about 900° C.
In the meantime, inside the carburizing furnace, sooting (a sooting phenomenon) occurs in the furnace due to a carburizing gas to be introduced during carburizing and the carburizing gas remaining after the carburizing. The sooting is likely to occur when the temperature becomes 700 to 800° C. in particular, and an adhesion amount of soot increases in the temperature zone.
As described previously, due to the surface temperature of the furnace inner side of the first heat insulator being 900° C., the surface temperature of the outer wall side of the first heat insulator becomes a temperature of 800° C. or less. That is, the surface temperature of the outer wall side of the first heat insulator becomes the temperature at which the sooting starts to occur. Therefore, in the conventional carburizing furnace, sooting has occurred between the first heat insulator and the heat insulator located on the further outer side of the first heat insulator (to be referred to as a “second heat insulator,” hereinafter).
When the sooting continues to occur between the first heat insulator and the second heat insulator, soot increases in thickness between the heat insulators. Thereby, the first heat insulator is pressed out toward the furnace inner side. The case where such a state continues as it is causes a risk that, of the first heat insulator, rising and falling off to the furnace inner side occur. Therefore, “burnout” that burns off the soot that has adhered between the heat insulators has been performed conventionally.